X-ray image converter using a high performance folded objective lens

ABSTRACT

A high performance folded objective lens and a compact fluoroscopic apparatus incorporating the lens are disclosed. The objective lens has a relative aperture of f/1.0 and its performance has been optimized for use with an X-ray image intensifier tube. The lens consists of two spaced groups with a fold of 90* introduced between the two groups. The first group, which consists of four elements, is of relatively low power. The second group which consists of six elements is of relatively high power, designed particularly to have a short physical length. Provision has been made for makeup glass in the back focal region. The fluoroscopic apparatus, of which the lens is a part, achieves compactness measured along the axis of the X-ray beam by folding the optical axis. In a preferred application, this feature permits location of the fluoroscopic apparatus in the limited vertical dimensions available beneath an examination table without reduction in the efficiency of optical coupling to the conventional multiple output devices.

-\ 5 O i f l 0 SR Liverpool, N.Y.;

Robert Godbarsen, Jr., Wauwatosa; John A. Bickford, Milwaukee, Wis. [211App]. No. 878,137 [22] Filed Nov. 19, I969 [45 I Patented Nov. 23, 1971[73] Assignee General Electric Company [54] X-RAY IMAGE CONVERTER USINGA IIIGII PERFORMANCE FOLDED OBJECTIVE LENS 8 Claims, 3 Drawing Figs.[52] US. Cl 250/77, 250/71.5 R, 250/71.5 S, 250/213 R, 350/171, 350/202[51] Int. Cl G0lt0l/20 [50] Field of Search 250/77, 71.5.7155, 213;350/202, 171 [56] References Cited UNITED STATES PATENTS 3,439,1144/1969 Taylor 250/77 3,515,870 6/1970 Marquis 250/77 X FOREIGN PATENTS1,462,444 1 H1966 France 350/202 Primary Examiner-James W. LawrenceAssistant Examiner-Morton J. F rome Attorneys-Richard V. Lang, Marvin A.Goldenberg, Joseph B. Fonnan, Frank L. Neuhauser and Oscar B. WaddellABSTRACT: A high performance folded objective lens and a compactfluoroscopic apparatus incorporating the lens are disclosed. Theobjective lens has a relative aperture of f/ 1.0 and its performance hasbeen optimized for use with an X-ray image intensifier tube. The lensconsists of two spaced groups with a fold of 90 introduced between thetwo groups. The first group, which consists of four elements, is ofrelatively low power. The second group which consists of six elements isof relatively high power, designed particularly to have a short physicallength. Provision has been made for makeup glass in the backfocalregion. The fluoroscopic apparatus, of which the lens is a part,achieves compactness measured along the axis of the X-ray beam byfolding the optical axis. In a preferred application, this featurepermits location of the fluoroscopic apparatus in the limited verticaldimensions available beneath an examination table without reduction inthe efficiency of optical coupling to the conventional multiple outputdevices.

CONVERTER 5 INTENSIFIER X-RAY IMAGE CONVERTER USING A HIGH PERFORMANCEFOLDED OBJECTIVE LENS BACKGROUND OF THE INVENTION 1. Field of theInvention The present invention relates to high performance lens designsof high relative apertures (typically f/ L) and is speciallycharacterized by having a good low frequency response. A compactfluoroscopic apparatus is also disclosed.

2. Description of the Prior Art There is a need for a lens optimized foruse in a fluoroscopic system employing an X-ray tube and an imageintensifier with television or film camera outputs. In fluoroscopicsystems for hospital use there is the prime requirement that patientexposure be minimized and to this end it is desirable that the lens beof large aperture. In systems of this type, where the image intensifieroutput screen provides the input source for the visual image, thelimiting resolution occurs in the image intensifier whose lineresolution maybe on the order of 20 to 25 line pairs/mm. In the designof optical components to relay the image from the intensifier tube tothe various output devices, the customary emphasis on high performanceat higher spacial frequencies, will not insure a good low frequencyresponse where the information content of the image is largelyconcentrated. Additionally, when location of the exit pupil is notoptimized in respect to the application of the lens or where nonoptimalplacement of the exit pupil is tolerated in order to use previouslyexecuted lens designs, there are often rather substantial losses inoff-axis illumination of the image, i.e. vignetting.

A further degradation in lens performance may also occur if due tovariations in image tube glass thickness the input image must proceedthrough additional layers of glass without compensation in the design.

In a typical application where the fluoroscopic system is employed forexamination of a patient upon an examination table, it is desirable thatthe table be of conventional height from the floor. Assuming that theX-ray source is disposed over the patient with its beam projectingdownward along a vertical axis, then under the patient and within thespace between the under surface of the examination table and the floor,one should be able to locate the remaining elements of the fluoroscopicsystem. If for instance, an image intensifier, an objective lens, a beamsplitter permitting a plurality of optical takeoffs, and a televisioncamera, which are the conventional components of a fluoroscopic system,are arranged along a vertical axis under the table, their verticalextension would substantially exceed the available vertical,under-the-table dimension. Assuming a necessity to keep the verticaldimension of the fluoroscopic system to a value compatible withconventional table heights, a fold in the axis of the fluoroscopicsystem is dictated. While folding the axis of the system will achieve amajor reduction in the vertical dimensions of the under-thetablecomponents, it does not completely solve the problem. Since asubstantial amount of the under-the-table vertical dimension is taken upby the image intensifier, it is desirable that any optical elementscoupled to the image intensifier be of minimum vertical dimensions. Thisarrangement and these dimensional requirements should be achieved in amanner not adversely affecting the optical performance of the system.

SUMMARY OF THE INVENTION present invention to provide an folded lens foruse in a fluoro- It is still another object of the present invention toprovide a folded lens arranged to couple an image in a fluoroscopicsystem to plural output optical devices with a minimum of vignetting.

It is a further object of the invention to provide a folded lens adaptedto use with varying amounts of glass in the back focal region withoutdegradation of the optical performance.

These and other objects of the invention are achieved in a fluoroscopicapparatus having a fold in the optical portions of the apparatus. Thefluoroscopic apparatus includes an X-ray source, an image intensifierplaced on the axis of the X-ray source beyond the object underexamination and a novel folded objective lends coupled to the imageintensifier which folds the optical axis orthogonally to the axis of theX-ray source. The folded lens is then arranged through a beam splitterto couple light selectively to a plurality of optical output devicessuch as a cine camera, still camera, and a closed circuit televisioncamera. The folded objective lens is of novel design and consists of twogroups separated by a mirror. The front group consists of four elementsin a Tessar type lens and is of a relatively long focal length. Thesecond group consists of six element resembling an infinite conjugatelens. Midway between the two lens groups a mirror is arranged to achievea fold in the optical axis of the lens. The folded lens has a makeupglass provision in the back focal region to accommodate a plurality ofglass thicknesses in the cover glass of the image intensifier. The exitpupil of the folded objective lens is placed well in front of the frontgroup. The plural optical output elements may be arranged about a beamsplitter with their entrance pupils at this exit pupil, which reducesvignetting to a minimum.

BRIEF DESCRIPTION OF THE DRAWING The novel and distinctive features ofthe invention are set forth in the claims appended to the presentapplication. The invention itself, however, together with the furtherobjects and advantages thereof may best be understood by reference tothe following description and accompanying drawings, in which:

FIG. 1 is an illustration partially in perspective of a folded objectivelens in accordance with the invention and disposed in a compactfluoroscopic apparatus;

FIG. 2 is a more detailed illustration of the lens itself; and

FIG. 3 is a graph illustrating the lens performance in an example havinga 90-min. focal length.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIG. 1, the foldedobjective lens is shown at 11 in a novel television aided fluoroscopicapparatus. The folded objective lens 11 couples an image produced by anX-ray image intensifier tube 12 to a television camera 13 and monitor l4and optionally through a mirror system or beam split- "'ter" 15, toacine camera" 16 or still camera 17. The X-ray image is initially fonnedon an input screen 18 of the image converter and intensifier tube 12located under the subject 20. As illustrated, the X-ray beam isprojected from an X-ray tube 19 downwardly through the subject andemergent rays, modulated in intensity by the variable opacity of thesubject, impinge on the input screen 18. The image converter andintensifier tube 12 forms an optical image of the X-ray image on itsoutput screen 21.

The image converter and intensifier tube 12 provides the optical inputfor the folded objective lens 11. The image converter and intensifiertube 12 is a vacuum device which converts a relatively large X-ray imageformed on its input screen 18 to a relatively small, more intenseoptical image at its output screen 21. Common image intensifiers haveinput screens of from 6 to l2 inches and have an output screen usuallyof 1 inch or less (15 to 20 mm. in typical systems) in diameter.Brightness may be increased by a factor of 3,000-l0,000 and the outputis usually of a yellow-green hue.

The image at the image intensifier output screen 21 is coupled by theobjective lens 11 to the above-mentioned output optical devices. Asillustrated in FIG. 1, these devices, including the television camerat3, the cine camera 16, the still camera 17, and the mirror system or"beam splitter" 15, which provides an output selection function, arearranged in a horizontal plane passing through the output axis of thefolded objective lens 11. This physical arrangement minimizes thebelow-patient height of the fluoroscopic apparatus.

The optical functions of the beam splitter 15 are perfonned by a glassplate having a reflective coating which reflects a portion of theimpinging light and transmits another portion. The division is afunction of receiver sensitivity, generally in the range of 50/50 to90/10 depending on application (90 percent reflected). The glass plateis translatable (by means not shown) to one of two mutually orthogonalpositions. in the illustration, the plane of the beam splitter isvertical in both positions and by means of the reflective surfaces, thebeam splitter optionally couples either to the cine camera 16 or to thestill camera 17. in either position, however, light is transmitteddirectly through the beam splitter to the television camera 13. One mayoptionally translate the glass plate to mutually orthogonal positionswhere one position is rotated about a horizontal axis from the otherposition. This will permit one of the optical output devices to bedisposed above rather than to one side of the beam splitter. In eitherdisposition, the camera is ordinarily oriented along its axis so thatthe patients head is up in the pictures.

it should be noted that in the illustration of FIG. 1, the outputelements 13, 16, 17 and the objective lens 11 are shown in a slightlyexpanded view for clarity in illustration. In practice, the beamsplitter may be regarded as occupying a space which is cubical with theedge dimension of the cubic space being equal to the distance from thefront group of the objective lens to it exit pupil'( 100 mm. in a 90 mm.example). The objective lens and each of the output lenses are thendisposed at each of the lateral or upper faces of this cube for aminimum vignetting position. This optimal disposition of the lensesreduces vignetting to approximately 25 percent. For certain outputdevices, optimal positioning is not essential but is available whendesired.

One may now consider the optical aspects of the objective lens 11 inrelation to the output optical devices. An input lens 22 of thetelevision camera 13 and input lenses of the cine and film cameras 16and 17 are arranged with respect to the objective lens 11 so as to havean "infinity" conjugate ratio. That is to say, the image intensifieroutput screen 21 is placed at the focal plane of the objective lens 11so as to produce essentially parallel light between the objective lens11 and the television camera lens 22 and similarly, the televisioncamera lens 22 is arranged such that the camera pickup tube target (notshown) within the television camera 13 is in the focal plane of the lens22, permitting the lens 22 to be focused to infinity. This is an optimumdesign position for all three optical output devices l3, l6 and 17. Theprovision of an infinity conjugate ratio creates a parallel wave frontbetween the lens 11 and the three optical output devices, and permitsgreater latitude in adjustment of the spacing between the respectivelenses. At the same time, the interposition of an optically flatbeamsplitting device 15 between the lenses for switching in the cine orstill cameras l6, l7, introduces a minimum of lens error.

The lenses 11 and 22 should be in relatively close mutual proximity toavoid the loss of light from elf-axis image points, i.e. vignetting. Thelens 11 has its exit pupil in front of its front element (100 aim. beingtypical for a lens of 90-mm. focal length). The combination, in onepractical example, exhibits a maximum of 25 percent vignetting when theentrance pupil of lens 22 is located at this position. Preferably theentrance pupils of lens 22 and of the other optical output devices areplaced at the exit pupil of the lens 11 with the penalty for greaterseparations being increased vignetting.

The performance requirements of the folded objective lens 111 areestablished by the following system components. To

minimize patient exposure to X-ray radiation the lenses 11 and 22 shouldhave relatively large apertures and should be efficiently coupled. Oneexample of the objective lens 11 has a 90-mm. focal length and anaperture of f/ l .0 corresponding to a lens diameter of 90 mm.Similarly, the television camera lens 22 should have a relatively largeaperture, typically from 170.75 to 171.0 and provide an entrance pupildiameter of the same approximately 90 mm. to intercept the image beam.

In the event that an interlens spacing exceeding the desired figure isrequired, it may be desirable that the output lens 22) have a smallerentrance pupil. Although this arrangement results in some reduction inoptical efliciency, it produces a more uniform illumination of theimage.

The maximum useful resolution in the system is ordinarily set by theimage intensifier tube 12, which in a typical case, has a resolution of20 line pairs/mm. in the plane of the output screen 21. The X-ray tubeitself may be capable of at least twice this resolution and the opticalelements are ordinarily of about four times this quality. The outputmonitoring requirements, assuming a standard 525 line television viewingsystem, establish a resolution requirement of approximately 25 linepairs/mm. at the output screen 21, while a camera output system (such as16 or 17) may be capable of as great resolution as the opticsthemselves. The foregoing resolution requirements thus place a premiumon objective lenses having good low frequency response and to achievethis end, the objective lens is designed to provide an image havingsubstantial contrast at all spacial frequencies between 0-40 linepairs/mm. The resulting cut-off frequency in this particular designvaries from 150 line pairs/mm. on axis to line pairs/mm. at the edge ofthe image. A graph of this performance property obtained from actualtest data on a -mm. example is illustrated in FIG. 3.

The lens 1 1 has been designed with 5,200 Angstrom units as the nominalcenter of the spectral range. This corresponds to a common value forimage intensifier tubes. The color correction of the lens however ismaintained over any reasonable broad bandwidth (approximately 2,000Angstrom units) within the visible spectrum suiting the lens for use ina wide variety of practical applications.

A final system requirement which this lens has been designed to meet isthat it be fully corrected for differing amounts of glass in the faceplates of the various kinds of image intensifier tubes with which it islikely to be used.

The folded objective lens 11 is illustrated in detail in FIG. 2. It is a10 element lens having four elements (A, B, C, and D) in a first orfront group separated by a mirror M from six element (E, F. G. H. J. K)forming a second or back group. The function of the mirror is to providea 90 fold in the axis of the lens. Both the first group (A, B, C, and D)and the second group (E, F, G, H, J, K) are convergent, with the firstgroup having relatively low power and functioning primarily to refractthe outer :most ray of the off-axis bundle sufficiently to get it withinthe transverse dimensions of the following lens assembly. Associatedwith the second group is a final plane member L of make-up glass.

Considering the lens elements one at a time, the elements A, B fonn acrown-flint doublet while the meniscus C, which has a negative power,and the meniscus D are crown elements. In the second group, element E,F, and H, J are both double tlints and the menisci G and K are alsoflints. The mirror M is disposed at 45 with respect to the axes of thefront and back groups of the objective lens and is conventionally afirst surface mirror having an aluminum reflecting layer. The mirror isoptically flat and is suitably coated to avoid deterioration and toenhance optical efficiency. Since minimum overall dimensions of theobjective lens are desirable the first and second groups of lensesclosely abut the cube which the mirror occupres.

As mentioned above, the front group is of relatively low power being ofapproximately l,0O0-mm. focal length while total lens may have a focallength of 90 mm. (in a 90-mm. example). The front group is of relativelylow speed (17 10.0) and tional lens design approaches.

Because of this dimensional requirement, the second group is designedwith the general philosophy of using all positive elements and offollowing each of the stronger elements with immediate correction.Approximately half the power is assigned to the first doublet (EF) adouble flint which is followed by a meniscus G for correction. A seconddouble flint (HJ) having substantial power is also provided. It isfollowed the image plane, the physical length of second proximately 102units for a l lO-unit focal length.

The final element shown in H6. 2 is a planoparallel window L, whichfunctions as a sheet of make-up glass. It may be treated as a part ofthe lens proper since retaining rings are ordinarily provided to supportit integrally with the other elements. It has a design" thickness of 6.7

without degradation. When the lens is used with such a tube of maximumthickness, no make-up glass is included in the lens assembly. If a tubehaving a lesser face plate thickness is used, a sheet of make-up glassis provided of such thickness as is required to make the total thicknessof glass between last element K and the image equal to the original"design" thickness.

The foregoing elements provide a highly corrected folded objective lenshaving an aperture off/L0 suitable for use in the fluoroscopic system sofar described. In the example referred to, where the focal length of thelens in 90 mm., the lens has an image format of mm. in diameter and anexit pupil 90 mm. in diameter located I00 mm. in front of the frontelement. The overall vignetting factor is significantly reduced due tothe strategic placement of the exit pupil of objective lens ll close tothe entrance pupil of the television camera lens 22. In the case of anormal design, the pupil would be located within the lens and for agiven positive off-axis point, the upper portion of the light bundlefrom the image would be vignetted within the lens 11. When this lightbundle encounters the television camera objective lens 22, the lowerportion of the bundle will ordinarily be additionally vignetted,resulting in a total vignetting factor on the order of 60 percent.Having the exit pupil remote from the lens as is the case for theobjective lens 11 herein considered, results in the unusual condilens 11is largely coincident with the vignctted loss the TV objectivc, whichalso vignets the lower portion of the bundle. Thus by a super positionof the exit pupil of the lens ll upon the entrance pupil of the TV lens22, the overall vignetting is reduced to percent.

In order to place the exit those rays which pass through the remote exitpupil rather than those which pass through an exit pupil internal to thelens assembly.

A table of the final lens design at a standardized equivalent focallength of 100 and a relative aperture off/1.0 is given below:

The tabulated figures are nominal dimensions for use in manufacturingand are subject to conventional manufacturing tolerances. The actualtested performance of the lens in a mm. focal length example isillustrated in FIG. 3 where the modulation transfer function is plottedalong a three-coordinate axis. The modulation transfer function whosemodulus is the vertical coordinate in FIG. 3 is a representation of theability of the lens to reproduce an object at varying spacialfrequencies whose intensity varies in a sinusoidal fashion. The abilityto reproduce such sinusoidal variations, which is graphed in FIG. 3, isa ratio of the modulation of the image relative to the modulation of theobject. it might be spoken of as the contrast ratios between the imageand the object. This property has been plotted against spacial frequencyand radial position along the object. In FIG. 3, the line 31 correspondsto an on-axis position; the lines 32 and 33 to a position 5 mm. offaxis; and the lines 34 and 35 to an off-axis position of 10 mm. Thesymbols R" and T illustrated on the line traces 32-35 show disparatetreatment for radial and tangentially oriented lines. This difference intreatment is an indication of nonsymmetrical point images. The modulusis plotted at each of the given object positions against the thirdcoordinate, namely the spacial frequencies in line pairs/mm. It may beseen that the graph illustrates measurement through the range of fromzero to 40 line pairs/mm.

From a consideration of this graph, it will be seen that the modulus ofthe optical transfer function is l at the origin (on object axis, atzero spacial frequency) and remains close to unity at low spacialfrequencies irrespective of the radial position on the object. As thespacial frequency increases from zero to 40 lines/mm, the functiondecreases, the decrease tending to becoming more marked as one moves offaxis along the object. If one takes the particular line frequency of 40lines/mm, the modulus falls from a value of approximately 0.70 toapproximately 0.50 at the S-mm. off-axis position and 0.35 at the 10-mm.position (averaging the R and T plots).

in H6. 3, the lateral chromatic aberration accounts for the reduction ofthe modulus for the tangential characteristic (33, 35) with respect tothe radial characteristic (32, 341). The lateral chromatic aberration isa radial aberration which has no affect on the modulus for radiallyoriented lines, but does have an affect on tangentially oriented lines.In the case of a small off-axis image, the lateral color error appearsas a small radial smear of progressively changing hue. If the linestructure of the image is radially oriented the modulus is unaffected,while if the line structure of the image is tangentially oriented,successive lines blur into one another and the modulus is reduced.

In the performance characteristic in FIG. 3, the radial characteristic(32) has a modulus value of approximately 0.57 at the -mm. objectposition at a spacial frequency of 40 lines/mm. At the lO-mm. objectposition at the same spacial frequency the radial characteristic (34)continues to have a modulus value of approximately 0.5, while thetangential characteristic (33) is now reduced to approximately 0.24.Taking the tangential line performance by itself, the performance of0.24 at 40 lines/mm. is still well in excess of the 0.10 figure which isusually regarded as marginal. At 20 lines/mm. the tangential resolutionis approximately 0.43, indicating that the design is quite conservativeat the intended upper spacial frequency limit of 20 lines/mm. Since mostsubject matter contains lines of random orientation, the subjectiveeffect is ordinarily viewed as a composite one, approximating an averageof the two individual radial and tangential properties.

The curves illustrated in FIG. 3 describe a lens having good lowfrequency performance whose cut-off frequency generally exceeds 80lines/mm. throughout the object positions and which has a verysubstantial modulus at 40 lines/mm. While the cutoff region is notgraphed in H6. 3, it represents the point at which the modulus falls bowa useful level (usually approximately 0.10 as noted above). The curvesin FIG. 3 thus denote a lens design emphasizing extremely good lowfrequency response not only throughout the design region of from 0 to 20lines/mm, but throughout the region of from 0 to 40 lines/mm.

The values indicated in H0. 3 represent good performance from a lensdesign standpoint and are in excess of those observed in competitivelenses of equal focal lengths and apertures. Typical values for themodulus encountered in comparable lenses measured at a spacial frequencyof 20 lines/mm, vary from 0.4 to 0.3 on the object axis to from 0.46 to0.0 at the at the mm. object position. The comparable values for thepresent lens of 0.85 on the object axis and ap proximately 0.57(averaging R and T characteristics) at the lO-mm. object position, thusrepresent a substantial improvement.

The above table provides the design data for a lens incorporating theinvention. Since the table is given in arbitrary units it is intendedfor the design of lenses over a range of focal lengths, of which atypical example is the 90-mm. focal length lens previously discussed.The lens design retains its high quality over a range of locations ofthe exit pupil. Consequently, one may employ the tabular values of theclear diameters of the elements wherein a number of the elements areoversized to place the exit pupil well in advance of the lens to achievea minimum of vignetting in the present coupled optics application. Ifsuch an application is not contemplated, however, and a moreconventionally located internal exit pupil is desired, the oversizedelements may be reduced to conventional values.

The lens design contemplates 6.7 units of the indicated variety of glassin the back focus of the lens between the last doublet and the objectplane. The actual placement of the makeup glass, when such glass isnecessary, is variable within the rather small limits of space remainingbetween the surface of the last doublet and the object plane.

In a fluoroscopic apparatus, the foregoing folded lens permits theoptical axis to be conveniently folded with respect to the axis of themake-up beam. Since the X-ray beam is downwardly directed, the requisiteelements of the fluoroscopic system required to be arranged under thepatient along the vertical axis are now reduced to the image intensifierwhich ordinarily requires about 15 inches and the folded objective lens,which in a -min. example occupies about 200 mm. (8 inches) of verticalspace to achieve the fold. The remaining optical elements, such as thebeam splitter 15, the cine camera 16, the still camera 17, and thetelevision camera 13, may be arranged horizontally in the region of orabove the folded objective lens. Thus, they need not extend very muchbelow the lower extreme of the objective lens. The same spacialadvantage is also achieved when the objective lens is used with simpleroptical systems, such as those involving only direct coupling to atelevision system. By this arrangement, the under-the-patientfluoroscopic apparatus is conveniently arranged under an examinationtable of conventional height.

What is claimed as new and desired to be secured by Letters Patent inthe United States is:

l. in a fluoroscopic apparatus adapted for use with an X-ray beam, thecombination comprising:

a. an image converter and intensifier coaxially oriented along saidbeam;

b. a folded objective lens having two groups with a mirror interposedbetween; one of said groups being a back group of relatively high powercoaxially arranged with said image intensifier and optically coupledthereto, and a second, front group of relatively low power, whose axisis folded to a position orthogonal to that of said rear group, saidmirror providing said fold in said axis, the exit pupil of said foldedlens being a substantial distance in front of said front group; and

c. output optical means including at least one lens having the entrancepupil thereof coupled to said folded lens at said exit pupil.

2. The combination set forth in claim 1 wherein said back group consistsof all positive doublet and singlet elements to achieve minimum physicallength, said physical length approximating the focal length thereof.

3. The combination set forth in claim 1 wherein means are provided forreflecting the output of said objective lens orthogonal to thehorizontal axis thereof, to one of said output means.

4. The combination set forth in claim 3 wherein at least one of theorthogonal positions is displaced from said axis in a horizontal plane.

5. Tile combination set forth in claim 3 wherein one of the orthogonalpositions is vertically displaced above said axis.

6. The combination set forth in claim 3 wherein said reflective means isa beam splitter partially reflecting the light coupled thereto to atleast one output means and partially transmitting the light coupledthereto to a second output means coaxial with said horizontal axis.

7. The combination set forth in claim 6 wherein the minimum transmissivepath length through said beam splitter from said front group to saidoutput means approximates said substantial distance to permit saidoutput means to be arranged substantially at said exit pupil to minimizevignetting.

8. The combination set forth in claim 3 wherein the minimum reflectivepath length via said reflecting means from said front group to saidoutput means approximates said substantial distance to permit saidoutput means to be arranged substantially at said exit pupil to minimizevignetting.

1. In a fluoroscopic apparatus adapted for use with an X-ray beam, thecombination comprising: a. an image converter and intensifier coaxiallyoriented along said beam; b. a folded objective lens having two groupswith a mirror interposed between; one of said groups being a back groupof relatively high power coaxially arranged with said image intensifierand optically coupled thereto, and a second, front group of relativelylow power, whose axis is folded to a position orthogonal to that of saidrear group, said mirror providing said fold in said axis, the exit pupilof said folded lens being a substantial distance in front of said frontgroup; and c. output optical means including at least one lens havingthe entrance pupil thereof coupled to said folded lens at said exitpupil.
 2. The combination set forth in claim 1 wherein said back groupconsists of all positive doublet and singlet elements to achieve minimumphysical length, said physical length approximating the focal lengththereof.
 3. The combination set forth in claim 1 wherein means areprovided for reflecting the output of said objective lens orthogonal tothe horizontal axis thereof, to one of said output means.
 4. Thecombination set forth in claim 3 wherein at least one of the orthogonalpositions is displaced from said axis in a horizontal plane.
 5. THecombination set forth in claim 3 wherein one of the orthogonal positionsis vertically displaced above said axis.
 6. The combination set forth inclaim 3 wherein said reflective means is a beam splitter partiallyreflecting the light coupled thereto to at least one output means andpartially transmitting the light coupled thereto to a second outputmeans coaxial with said horizontal axis.
 7. The combination set forth inclaim 6 wherein the minimum transmissive path length through said beamsplitter from said front group to said output means approximates saidsubstantial distance to permit said output means to be arrangedsubstantially at said exit pupil to minimize vignetting.
 8. Thecombination set forth in claim 3 wherein the minimum reflective pathlength via said reflecting means from said front group to said outputmeans approximates said substantial distance to permit said output meansto be arranged substantially at said exit pupil to minimize vignetting.